Long haul transport of Ethernet is growing at 10 Gb/s and at 40 Gb/s. One of the preferred transport formats is specified in ITU-T G.709 Optical Transport Network (OTN). OTN specifies 110 Gb/s Ethernet LAN-PHY transport as Generic Frame Protocol (GFP) mapped into an asynchronous Optical Channel Transport Unit of level 2 (OTU2). However, network operators typically prefer a 100% bit transparent format. Several synchronous constant bit rate mappings have emerged that use the OTU2 Constant Bit Rate (CBRx-b) mapping but result in a higher than standard OTU2 bit rate (such as, 11.05 Gb/s from CIENA Corp. and Siemens, and 11.1 Gb/s from Cisco Systems). Another accepted method uses the standard asynchronous GFP mapping method but also carries the Ethernet preambles in reserved Optical Channel Payload of unit k (OPUk) overhead to provide some transparency (but not 100% bit-transparency) at the standard 10.7 Gb/s rate (such as CIENA Corp.). The move to 40 Gb/s transport uses the Optical Channel Transport Unit of level 3 (OTU3) format. At this time Ethernet is either carried as Packet over SONET (POS) at OC-768 and mapped into an OTU3 using the Regenerator Section level n (RSn) mapping or multiplexed as four 10 Gb/s Ethernet tributaries, each mapped into an Optical Channel Data Unit of level 2 (ODU2), into an OTU3 using the standard ODU[i]j mapping defined in G.709. For example, Stratalight Communications of Los Gatos, Calif. provides a multiplexer that maps four 10 G Ethernet LAN-PHY tributaries into ODU2's using the semi-transparent GFP plus preamble method and then multiplexes them into a proprietary OTU3. As before with OTU2, carriers prefer and will ask for 100% bit transparent multiplexing of four 10 G Ethernet LAN-PHY tributaries to an OTU3 even if it is at a slightly higher than standard rate. NTT Electronics Corporation (NEL) provides an integrated circuit that can be used to build a multiplexer for quad 100% bit transparent 10 G multiplexing to a higher than standard rate OTU3 and supports ±100 PPM (part per million) tributaries.
Transport carriers, such as long-haul providers, prefer 100% bit transparent transport of 10 Gb/s Ethernet LAN-PHY signals for various reasons. For instance, transparent transport provides simpler provisioning providing a connection that emulates fiber. Another reason is that some routers use the preamble and 64B/66B code words for proprietary control and data channels and dropping those bits disrupts router-to-router communications. The 10 Gb/s Ethernet LAN-PHY interface transmits at a rate of 10.3125 Gb/s±100 PPM. The G.709 OTUk asynchronous mapping (CBRx-a) provides justification opportunities to carry ±20 PPM payloads. As a result only synchronous mappings exist for 100% bit transparent 10 G Ethernet LAN-PHY transport. The asynchronous multiplexing (ODU[i]j) provides ±20 PPM per tributary signal.
Disadvantageously, there is not support for any of the various semi-transparent and 100% transparent 10 Gb/s Ethernet LAN-PHY (10 GbE) transport methods in the ITU-T OTN specifications because these do not fit with the OTUk and ODU[i]j models. The GFP plus Preamble forces data into unused overhead locations. The 100% bit transparent methods require higher than standard clock rates for OTU2-LAN signals and when multiplexed will require higher than standard OTU3 rate and other changes to include 100 PPM offsets.
Averaged over time the OTN Asynchronous mapping method provides ±45 PPM and asynchronous multiplexing ±75 PPM of client offset while allowing ±20 PPM of aggregate OTU3 offset (this does not provide much buffer for large short term variations). The NEL approach asynchronously maps the 10 GbE into an ODU2, but the ODU2 clock generator must vary the ODU2 clock to make up for client offset beyond ±45 PPM. The ±45 PPM ODU2's are asynchronously multiplexed into the ODU3. The ODU2 clock generator is complex and de-multiplexing the 10 GBE signals requires asynchronously de-multiplexing and asynchronously de-mapping complicating the 10 GbE TX clock generator. This method does not support multiplexing 100% transparent OTU2-LAN signals (11.05 Gb/s and 11.1 Gb/s) generated by synchronously mapping (CBRx-b) 10 Gb/s Ethernet Signals that are preferred by network operators for handoffs.
The Stratalight multiplexer does not provide multiplexing of 100% bit transparent 10 GbE and does not support multiplexing of OTU2-LAN rate tributaries. Although the NEL framer supports multiplexing of 100% bit rate transparent 10 GbE, it does not support multiplexing OTU2-LAN rate signals. Both existing systems and methods require OTU2-LAN rate signals be terminated to 10 GbE LAN-PHY before being multiplexed.
In the past 10 GbE links were used to inter-connect core routers internal to the carrier's networks, but now carriers are offering 10 GbE handoffs to commercial customers. As these proliferate, the need to carry these in 100% transparent OTU2-LAN rate signals will grow as will the need to 100% transparently multiplex these signals to 40 Gb/s and 100 Gb/s.